1. Technical Field
This invention relates generally to techniques for efficiently using laser diodes in laser writers such as printers and recorders, and more particularly to precision laser drivers for such printers.
2. Background Art
In laser writers such as laser thermal printers and recorders, the optical power of lasers is used to affect a transfer of a dye or ink from a donor medium to a receiver member. High power is needed to enhance the writing speed of lasers. One technique for achieving high power in a laser thermal printer is to use a number of independently modulated diode lasers. U.S. Pat. No. 5,109,460, which issued to Baek et al. on Apr. 28, 1992, discloses an optical fiber print head system in which a plurality of independently-modulated diode lasers are each coupled to one end of a respective optical fiber. The other end of each of the optical fibers is placed in a grooved mount such that the fiber ends form a line of closely spaced elements forming a monolithic optical fiber head. The optical fiber head is imaged onto a thermal dye media as an array of closely spaced laser light spots.
U.S. Pat. No. 4,743,091, which issued to Gelbart on May 10, 1988, discloses an optical data storage apparatus in which numerous independently-modulated diode lasers are arranged as a closely packed, two dimensional array. The array is imaged onto a moving optical recording medium.
Precision laser drivers, such as drivers that maintain better than 0.5% current regulation, should be able to have the current which is commanded to a write laser remain at the commanded value regardless of time, temperature, etc. There are a number of known ways to produce such precision laser drivers; such as the so-called "balanced" system. In a balanced system, D.C. current from a constant current source is shuttled between the write laser and an associated "dummy" or dump laser. In the prior art, this process is used to keep the heat of the laser constant. In one known application, ten write lasers and ten associated dump lasers are used, so the current is shuttled between selected ones of the write laser to turn them ON or to the associated dump laser to turn the write laser OFF.
If the line from the current source to the laser is shorter than the quarter wavelength of the highest frequency, the line will appear as an unmatched load inductance for the whole range of frequencies used. The load inductance can produce a counter e.m.f. opposing the rise of current needed to shuttle the constant current source back and forth between the dump laser and the write laser.
In the case of low power lasers, merely matching the impedance of the load to the impedance of the line would effectively eliminate the effect on the load inductance. However, without using resistors, it is very difficult to get a broad band match; while the use of resistors results in power lost to heat. Thus, it is impractical to try to match the load to the line when using high power lasers.